Hydrophilic and corrosion resistant fuel cell components

ABSTRACT

One embodiment disclosed includes a product comprising a fuel cell bipolar plate comprising a substrate comprising a first face, a reactant gas flow field defined in the first face, and a layer over at least a portion of the first face, wherein the layer comprises a zeolite.

TECHNICAL FIELD

The disclosure generally relates to fuel cell components, such asbipolar plates, having hydrophilic and corrosion resistant properties.

BACKGROUND

Hydrogen is a very attractive fuel because it is clean and can be usedto efficiently produce electricity in a fuel cell. The automotiveindustry expends significant resources in the development of hydrogenfuel cells as a source of power for vehicles. Fuel cell vehicles aremore efficient and generate fewer emissions than today's vehiclesemploying internal combustion engines.

A hydrogen fuel cell is an electrochemical device that includes an anodeand a cathode with an electrolyte therebetween. The anode receiveshydrogen-rich gas or pure hydrogen and the cathode receives oxygen orair. The hydrogen gas is dissociated in the anode to generate freeprotons and electrons. The protons pass through the electrolyte to thecathode. The protons react with the oxygen and the electrons in thecathode to generate water. The electrons from the anode cannot passthrough the electrolyte, and thus are directed through a load to performwork before being sent to the cathode. The work may be used to operate avehicle, for example.

Proton exchange membrane (PEM) fuel cells are popular for vehicleapplications. The PEM fuel cell generally includes asolid-polymer-electrolyte proton-conducting membrane, such as aperfluorosulfonic acid membrane. The anode and cathode typically includefinely divided catalytic particles, usually platinum (Pt), supported oncarbon particles and mixed with an ionomer. The catalytic mixture isdeposited on opposing sides of the membrane. The combination of theanode catalytic mixture, the cathode catalytic mixture, and the membranedefine a membrane electrode assembly (MEA). MEAs are relativelyexpensive to manufacture and require certain conditions for effectiveoperation. These conditions include proper water management andhumidification, and control of catalyst poisoning constituents, such ascarbon monoxide (CO).

Several fuel cells are typically combined in a fuel cell stack togenerate the desired power. For the automotive fuel cell stack mentionedabove, the stack may include about two hundred or more bipolar plates.The fuel cell stack receives a cathode reactant gas, typically a flow ofair forced through the stack by a compressor. Not all of the oxygen isconsumed by the stack and some of the air is output as a cathode exhaustgas that may include liquid water as a stack by-product. The fuel cellstack also receives an anode hydrogen reactant gas that flows into theanode side of the stack.

The fuel cell stack includes a series of flow field or bipolar platespositioned between the several MEAs in the stack. The bipolar platesinclude an anode side and a cathode side for adjacent fuel cells in thestack. Anode gas flow channels are provided on the anode side of thebipolar plates that allow the anode gas to flow to the anode side of theMEA. Cathode gas flow channels are provided on the cathode side of thebipolar plates that allow the cathode gas to flow to the cathode side ofthe MEA. The bipolar plates may also include flow channels for a coolingfluid.

The bipolar plates are typically made of a conductive material, such asstainless steel, titanium, aluminum, polymeric carbon composites, etc.,so that they conduct the electricity generated by the fuel cells fromone cell to the next cell and out of the stack. Metal bipolar platestypically have a natural oxide on their outer surface that makes themresistant to corrosion. However, this oxide layer is not conductive, andthus increases the internal resistance of the fuel cell, reducing itselectrical performance.

As is well understood in the art, typically the membranes within a fuelcell need to have a certain relative humidity so that the ionicresistance across the membrane is low enough to effectively conductprotons. During operation of the fuel cell, moisture from the MEAs andexternal humidification may enter the anode and cathode flow channels.At low cell power demands, typically below 0.2 A/cm², water accumulateswithin the flow channels because the flow rate of the reactant gas istoo low to force the water out of the channels. As the wateraccumulates, it forms droplets that continue to expand because of thehydrophobic nature of the plate material. The contact angle of the waterdroplets is generally about 90° in that the droplets form in the flowchannels substantially perpendicular to the flow of the reactant gas. Asthe size of the droplets increases, the flow channel is closed off, andthe reactant gas is diverted to other flow channels because the channelsflow in parallel between common inlet and outlet manifolds. Because thereactant gas may not flow through a channel that is blocked with water,the reactant gas cannot force the water out of the channel. Those areasof the membrane that do not receive reactant gas as a result of thechannel being blocked will not generate electricity, thus resulting in anon-homogenous current distribution and reducing the overall efficiencyof the fuel cell. As more and more flow channels are blocked by water,the electricity produced by the fuel cell decreases, where a cellvoltage potential less than 200 mV is considered a cell failure. Becausethe fuel cells are electrically coupled in series, if one of the fuelcells stops performing, the entire fuel cell stack may stop performing.

In a PEM fuel cell environment, the bipolar plates are in constantcontact with highly acidic solutions (pH 3-5) containing F⁻, SO₄ ⁻, SO₃⁻, HSO₄ ⁻, CO₃ ⁻, and HCO₃ ⁻, etc. Moreover, the cathode operates in ahighly oxidizing environment, being polarized to a maximum of about +1 V(versus the normal hydrogen electrode) while being exposed topressurized air. Finally, the anode is constantly exposed to atmospherichydrogen. Hence, contact elements made from metal must be resistant toacids, oxidation, and hydrogen embrittlement in the fuel cellenvironment. As few metals exist that meet this criteria, contactelements have often been fabricated from large pieces of graphite whichis corrosion-resistant, and electrically conductive in the PEM fuel cellenvironment. However, graphite is quite fragile, and quite porous makingit extremely difficult to make very thin gas impervious platestherefrom.

Lightweight metals, such as aluminum and its alloys, have also beenproposed for use in making fuel cell contact elements. Such metals aremore conductive than graphite and can be formed into very thin plates.However, such light weight metals are susceptible to corrosion in thehostile PEM fuel cell environment. In light of the corrosion sensitivityof lightweight metals, efforts have been made to develop protectivecoatings. Yet some of these protection methods increase the electricalresistance of the aluminum plate to unacceptable levels. Other methodsof protection keep the conductivity at an acceptable level, but do notsufficiently achieve the desired level of protection.

SUMMARY OF EXEMPLARY EMBODIMENTS OF THE INVENTION

One embodiment disclosed includes a product comprising a fuel cellbipolar plate comprising a substrate comprising a first face, a reactantgas flow field defined in the first face, and a layer over at least aportion of the first face, wherein the layer comprises a zeolite.

Other exemplary embodiments of the invention will become apparent fromthe detailed description provided hereinafter. It should be understoodthat the detailed description and specific examples, while indicatingthe exemplary embodiments of the invention, are intended for purposes ofillustration only and are not intended to limit the scope of theinvention.

BRIEF DESCRIPTION OF THE DRAWINGS

Exemplary embodiments of the invention will become more fully understoodfrom the detailed description and the accompanying drawings.

FIG. 1 illustrates a broken-away, cross-sectional view of a bipolarplate for a fuel cell including a hydrophilic and corrosion resistantlayer, according to one embodiment of the invention;

FIG. 2 illustrates a broken-away, cross-sectional view of a bipolarplate for a fuel cell including a hydrophilic and corrosion resistantlayer over the channels of the bipolar plate, according to anotherembodiment of the invention;

FIG. 3 illustrates a broken-away, cross-sectional view of a fuel cellthat includes bipolar plates having a hydrophilic and corrosionresistant layer, according to one embodiment of the invention;

FIG. 4A illustrates a drop of water placed on a stainless steelsubstrate without a coating;

FIG. 4B illustrates a drop of water placed on a stainless steelsubstrate having a layer comprising a zeolite, according to oneembodiment of the invention.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

The following description of the embodiments is merely exemplary innature and is in no way intended to limit the invention, itsapplication, or uses. In one embodiment, zeolite coatings may benefit aPEM fuel cell operation by improving the corrosion resistance and watertransport in bipolar plate channels.

According to one embodiment of the invention, illustrated in FIG. 1, aproduct 10 comprises a substrate 14. In one embodiment, the substrate 14may be a bipolar plate 12 having a first face 42 and a second face 44.The bipolar plate 12 may include two sheets 40 and 20. The two sheets 40and 20 may be machined or stamped and are typically welded together. Areactant gas flow field comprising flow channels 16 and lands 18 isdefined in the first face 42. The channels 16 comprise sidewall(s) 30and bottom wall 32. Cooling fluid flow channels 26 may be provided overthe second face 44. In one embodiment, the substrate 14 comprises aconductive material, for example stainless steel, titanium, aluminum, orpolymeric composites including electrically conductive materials such ascarbon fibers or the like. A layer 28 is provided over at least aportion of the first face 42. The layer 28 may be hydrophilic andcorrosion resistant. In one embodiment, the layer 28 comprises azeolite. The zeolite may include an aluminosilicate tetrahedralframework, ion-exchangeable large cations, and a stablethree-dimensional structure.

Zeolite coatings are hydrophilic stable coatings and good ion exchangeresins. In one embodiment, a zeolite layer is constructed and arrangedin a manner sufficient to provide a hydrophilic stable coating in a fuelcell to improve water management. Zeolite coatings may be stable at alow pH. Zeolites may function to minimize the contamination of the MEAby metal cations, for example fluoride ions. In one embodiment, azeolite layer is constructed and arranged in a manner sufficient to trapor capture metal cations that leach out of the bipolar plates or othercomponents of the fuel cell stack. Metal cations may be produced in thefuel cell environment because of degradation of the MEA. Metal cationsmay reduce performance of the fuel cell stack by increasing theresistance of the MEA, impairing the stability of the membrane throughchemical degradation, and/or impacting the ion exchange capacity of themembrane.

In one embodiment, the layer 28 comprises a zeolite comprising aluminumoxide, silicon oxide, or a mixture thereof. The layer 28 may alsocomprise at least one zeolite powder impregnated inside at least onebinder such as a polymer resin. A suitable polymer resin may include awater insoluble polymer that can be formed into a thin adherent film andthat can withstand the hostile oxidative and acidic environment of thefuel cell. Examples of a suitable polymer resin include an epoxide,polyamide-imide, polyether-imide, polyphenole, fluoroelastomer (e.g.,polyvinyldene fluoride), polyester, phenoxy-phenolic, epoxide-phenolic,acrylic, and urethane. In one embodiment, the polymer resin may be across-linked polymer, for example a polyamide-imide thermosettingpolymer, and may be employed for producing impermeable coatings.

In one embodiment, the layer 28 may comprise a zeolite mixed with atleast one of carbon fibers, carbon nanotubes, carbon black, graphite,gold, silver, carbides, or nitrides. The carbon black or graphite mayhave a particle size ranging from about 0.05 micron to about 3 microns.In one embodiment, the layer 28 may be formed over the channels and mayinclude about 100 weight percent zeolite. In one embodiment, the layer28 may be sprayed and may include about 5 to about 50 weight percentzeolite. In one embodiment, the layer 28 may comprise about 30 to about70 weight percent binder. Suitable binders include those describedabove. In one embodiment, the layer 28 may comprise about 5 to about 70weight percent carbon.

In one embodiment, the zeolite may be the zeolite “ZSM-5,” analuminosilicate zeolite with a high silica and low aluminum content. Forexample, the SiO₂/Al₂O₃ mole ratio may be in the range of 15-100, 25-30,or 50-55. The structure of the ZSM-5 zeolite is based on channels withintersecting tunnels. The aluminum sites are very acidic. Thesubstitution of Al³⁺ in place of the tetrahedral Si⁴⁺ silica requiresthe presence of an added positive charge. When this is H⁺, the acidityof the zeolite is very high. The ZSM-5 may have a surface area of400-425 m²/g. In one embodiment, the zeolite may be made by ahydrothermal process. In one embodiment, the zeolite may be chemicallystable, porous, and hydrophilic. The zeolite may have a pore sizeranging from about 0.4 nanometers to about 10 nanometers and a contactangle ranging from about 0 to about 20 percent.

In one embodiment, the layer 28 may have a minimum thickness of about 40nanometers. In another embodiment, the thickness of the layer 28 mayrange from about 100 nanometers to about 500 nanometers. In yet anotherembodiment the thickness of layer 28 may range from about 0.5 micron toabout 2 microns.

The layer 28 may be formed over the bipolar plate 12 by any suitabletechnique including, but not limited to, physical vapor depositionprocesses, chemical vapor deposition (CVD) processes, thermal sprayingprocesses, sol-gel, spraying, dipping, brushing, spinning on, or screenprinting. Suitable examples of physical vapor deposition processesinclude electron beam evaporation, magnetron sputtering, and pulsedplasma processes. Suitable chemical vapor deposition processes includeplasma enhanced CVD and atomic layer deposition processes. CVDdeposition processes may be more suitable for the thin film layers ofthe layer 28.

Referring now to FIG. 2, the layer 28 may be selectively provided onlyover sidewalls 30 and bottom wall 32 of channels 16. This may be done,for example, by selectively depositing a mask over the lands 28 ofbipolar plate 12. Thereafter the layer 18 may be formed over at leastthe exposed portions of the first face 42 of bipolar plate 12 to limitthe area of formation of the layer to a non-masked portion. The mask maysubsequently be removed.

In another embodiment of the invention illustrated in FIG. 3, a product70 comprises a fuel cell 72. The product 70 comprises a first bipolarplate 62 having a first face 76 having a reactant gas flow field definedtherein by a plurality of lands 78 and channels 80, a second bipolarplate 64 having a first face 76 having a reactant gas flow field definedtherein by a plurality of lands 78 and channels 80, and a soft goodsportion 82 provided therebetween. The soft goods portion 82 may includea polymer electrolyte membrane 48 comprising a first face 84 and asecond face 86. A cathode 54 may overlie the first face 84 of thepolymer electrolyte membrane 48. A first gas diffusion media layer 50may overlie the cathode 54, and optionally a first microporous layer 52may be interposed between the first gas diffusion media layer 50 and thecathode 54. The first bipolar plate 62 overlies the first gas diffusionmedia layer 50. An anode 56 may underlie the second face 86 of thepolymer electrolyte membrane 48. A second gas diffusion media layer 60may underlie the anode layer 56, and optionally a second microporouslayer 58 may be interposed between the second gas diffusion media layer60 and the anode 56. The second bipolar plate 64 may overlie the secondgas diffusion media layer 60.

FIG. 4A illustrates a drop of water 200 placed on stainless steel 202without a coating. FIG. 4B illustrates a drop of water 200 placed onstainless steel 202 having a zeolite layer 28 according to oneembodiment of the invention. The zeolite layer 28 significantly reducesthe water contact angle. The water contact angle of the zeolite layer 28may range from about 0 to about 20 percent.

When the terms “over”, “overlying”, “overlies” or the like are usedherein with respect to the relative position of layers to each other,such shall mean that the layers are in direct contact with each other orthat another layer or layers may be interposed between the layers.

The description of the invention is merely exemplary in nature and,thus, variations thereof are not to be regarded as a departure from thespirit and scope of the invention.

1. A product comprising: a fuel cell substrate comprising a first faceand a layer over at least a portion of the first face, wherein the layercomprises at least one zeolite.
 2. A product as set forth in claim 1wherein the zeolite has a pore size of about 0.4 nanometer to about 10nanometers.
 3. A product as set forth in claim 1 wherein the zeolite hasa contact angle of about 0 to about 20 percent.
 4. A product as setforth in claim 1 wherein the substrate comprises a conductive material.5. A product as set forth in claim 1 wherein the substrate is selectedfrom the group consisting of stainless steel, titanium, aluminum, and apolymeric composite including an electrically conductive material.
 6. Aproduct as set forth in claim 1 wherein the layer further comprisesaluminum oxide.
 7. A product as set forth in claim 1 wherein the layerfurther comprises silicon oxide.
 8. A product as set forth in claim 1wherein the layer further comprises aluminum oxide and silicon oxide. 9.A product as set forth in claim 1 wherein the layer further comprises atleast one zeolite powder impregnated inside at least one binder.
 10. Aproduct as set forth in claim 9 wherein the binder comprises at leastone of a polyamide-imide, epoxide, polyether-imide, polyphenole,fluoroelastomer, polyvinyldene fluoride, polyester, phenoxy-phenolic,epoxide-phenolic, acrylic, urethane, cross-linked polymer, orpolyamide-imide thermosetting polymer.
 11. A product as set forth inclaim 1 wherein the layer further comprises carbon fibers.
 12. A productas set forth in claim 1 wherein the layer further comprises carbonnanotubes.
 13. A product as set forth in claim 1 wherein the layerfurther comprises carbon black.
 14. A product as set forth in claim 13wherein the carbon black has a particle size ranging from about 0.05micron to about 3 microns.
 15. A product as set forth in claim 1 whereinthe layer further comprises graphite.
 16. A product as set forth inclaim 15 wherein the graphite has a particle size ranging from about0.05 micron to about 3 microns
 17. A product as set forth in claim 1wherein the layer further comprises gold.
 18. A product as set forth inclaim 1 wherein the layer further comprises silver.
 19. A product as setforth in claim 1 wherein the layer further comprises carbides.
 20. Aproduct as set forth in claim 1 wherein the layer further comprisesnitrides.
 21. A product as set forth in claim 1 wherein the layerfurther comprises at least one of carbon fibers, carbon nanotubes,carbon black, graphite, gold, silver, carbides, or nitrides.
 22. Aproduct as set forth in claim 1 wherein the layer further comprisesabout 5 to about 70 weight percent carbon.
 23. A product as set forth inclaim 1 wherein the layer further comprises about 30 to about 70 weightpercent binder.
 24. A product as set forth in claim 23 wherein thebinder comprises at least one of a polyamide-imide, epoxide,polyether-imide, polyphenole, fluoroelastomer, polyvinyldene fluoride,polyester, phenoxy-phenolic, epoxide-phenolic, acrylic, urethane,cross-linked polymer, or polyamide-imide thermosetting polymer.
 25. Aproduct as set forth in claim 1 wherein the layer comprises about 100weight percent zeolite.
 26. A product as set forth in claim 1 whereinthe layer comprises about 5 to about 50 weight percent zeolite.
 27. Aproduct as set forth in claim 1 wherein the zeolite is a high silicaZSM-5 zeolite.
 28. A product as set forth in claim 1 wherein the layerhas a minimum thickness of about 40 nanometers.
 29. A product as setforth in claim 1 wherein the layer has a thickness of about 40nanometers to about 2 microns.
 30. A product as set forth in claim 1wherein the substrate comprises a bipolar plate having a reactant gasflow field defined in the first face.
 31. A product as set forth inclaim 30 further comprising: a plurality of said bipolar plate; and asoft goods portion positioned between adjacent bipolar plates and facingthe reactant gas flow fields, wherein the soft goods portion comprisesan anode and a cathode on opposite faces of a polymer electrolytemembrane.
 32. A product as set forth in claim 31 wherein the layer isconstructed and arranged in a manner sufficient to provide a hydrophilicstable coating.
 33. A product as set forth in claim 31 wherein the layeris constructed and arranged in a manner sufficient to capture metalcations that leach out of the bipolar plates.
 34. A process comprising:forming a layer over at least a portion of a fuel cell substrate,wherein the layer comprises at least one zeolite.
 35. A process as setforth in claim 34 wherein the forming a layer comprises at least one ofphysical vapor deposition, chemical vapor deposition, thermal spraying,sol-gel, spraying, dipping, brushing, spinning on, or screen printing.36. A process as set forth in claim 34, further comprising providing aplurality of said substrate having said layer over a portion thereof andproviding a soft goods portion positioned between adjacent substrate,wherein the soft goods portion comprises an anode and a cathode onopposite faces of a polymer electrolyte membrane.
 37. A process as setforth in claim 34 wherein the layer further comprises electricallyconductive particles.
 38. A process as set forth in claim 37 wherein theelectrically conductive particles comprise at least one of carbon blackor graphite.
 39. A process comprising: forming a zeolite by ahydrothermal process; and depositing the zeolite over at least a portionof a fuel cell substrate.